You cannot get the component upon the declaration of the variable (it might not be ready at that time). What you need to do is retrieve the component during the Awake phase:

@script RequireComponent (Health)

// declaration
var health : Health;

// retrieve component
function Awake()
{
    health = GetComponent(Health);
}

// use it
function OnGUI()
{
    GUI.Label(Rect(10,10,100,20), "Health:" + health.curHealth + "/" + health.maxHealth);
}

hp = GetComponent("Health") as Health; //do this at initialization

void OnGUI() {

GUI.Label(Rect(10,10,100,20), "Health:" + hp.curHealth + "/" + hp.maxHealth);

}